The present invention relates to a process and apparatus for electromagnetic tomography and more precisely an emitter adapted to use such a process.
It is known that the technique of electromagnetic tomography consists in emitting, from an emitter ordinarily disposed in a subterranean cavity, and particularly a bore hole, a magnetic field of high value whose emitted radiation is detected by means of a receiver disposed in an adjacent region. On the one hand, upon varying the magnetic field, and on the other hand, by displacing the emitter and/or the receiver so as to cause the portion of the soil between these two elements to vary, it is possible to construct a map representing the nature of the soils existing between the emitter and the receiver.
The magnetic field emitters used in this type of technique are ordinarily constituted essentially by an alternating current generator supplying a solenoid with a magnetic core. The efficiency of such emitters is directly connected to the intensity of the magnetic field which they are adapted to produce. Moreover, the magnetic field produced by a solenoid traversed by a current is proportional to the product of the apparent permeability .mu. of its magnetic core, by the number N of its windings, by the cross section S of these latter and by the intensity I of the current which passes through it (M=.mu..N.S.I).
Under these circumstances, it is known in theory that the maximum magnetic field supplied by the emitter is obtained when each of these mentioned parameters has a maximum value.
The parameter of magnetic permeability .mu. is in direct relation to the nature of the material constituting the solenoid core, and its maximum value is correspondingly limited by the inherent nature of this material.
As to the current I, it is known that the value of this latter is rapidly limited, particularly because of the specific characteristics of the cables ensuring the supply of the emitter, and of the heating produced within the latter which, if it becomes too great, is likely to damage the circuits and the electric components used.
The cross section S of the windings constituting the solenoid and the number N of the latter are limited by the diameter of the cross section which it is desired to give to the emitter, which cross section is dictated by the small dimensions of certain bore holes.
Furthermore, the inherent principle of the tomographic methods used dictates that, for each of the relative positions of the emitter and the receiver, there is created a series of magnetic fields over a range of predetermined different frequencies. As a result, it is thus necessary that the magnetic field produced by the emitter maintains its power over all this range of frequencies.
Furthermore, it is interesting, particularly as to the time gain effected upon making the measurement, to work with multifrequencies, which is to say by generating a magnetic field at a base frequency and at harmonic frequencies which are odd or even. It is known that such a result is obtained by supplying the solenoid with a current whose signal shape is as close as possible to a square signal. The generators adapted to supply solenoids with current will therefore be able, in this embodiment, to be adapted to deliver such a square signal.
Moreover, the emitters used conventionally in the practice of electromagnetic tomographic processes are supplied with alternating current from the surface of the ground, via cables of great length. There results a certain number of substantial drawbacks.
In the first place, the inherent resistance of the cables and their great length lead to a marked drop of voltage in these, such that, if it is desired to avoid supplying only a low voltage to the emitter, it is necessary to provide an excessive level of supply voltage. However, the supply cables used do not always permit absorbing such a high voltage, particularly as it is then necessary to correspondingly over-dimension the cross section of these cables, which on the one hand raises the cost and on the other hand correspondingly increases the weight and size of the device.
Secondly, these supply cables carrying a low frequency alternating electric current are themselves a source of magnetic radiation, which disturbs the results of the measurement taken.
These various concerns are such that the emitters of the prior art in this field either are capable of emitting a magnetic field of sufficient power, but then have a large diameter such that this measurement technique can be used only in certain types of large diameter bore holes, or have diameters sufficiently small to permit their use in all types of bores, but the magnetic field that they produce is then too low for the measurement to be truly effective.